I'm assuming you mean regular coke, if so, I don't eat any refined sugars on non-cheat days. Once a week when I cheat, I'd much rather have some chocolate! I don't like regular soda enough to make it "worth" the empty calories.
Check out Warlobo's thread "White Death" for info on sugar - on the Woman's board (not sure if it's elsewhere also).
As for diet soda, I have maybe 2-4 a week. I'm a BIG TIME sugar addict, & I find that, instead of it curing my cravings, it spikes me to want MORE & REAL sweets - not good. I like green tea.
I also like the diet coke with lemon.